ABSTRACT The Popigai (100 km in diameter) and the Chesapeake Bay (40–85 km diameter) impact structures formed within ~10–20 k.y. in the late Eocene during a 2 m.y. period with enhanced flux of 3 He-rich interplanetary dust to Earth. Ejecta from the Siberian Popigai impact structure have been found in late Eocene marine sediments at numerous deep-sea drilling sites around the globe and also in a few marine sections outcropped on land, like the Massignano section near Ancona in Italy. In the Massignano section, the Popigai layer is associated with an iridium anomaly, shocked quartz, and abundant clinopyroxene-bearing (cpx) spherules, altered to smectite and flattened to “pancake spherules.” The ejecta are also associated with a significant enrichment of H-chondritic chromite grains (>63 μm), likely representing unmelted fragments of the impactor. The Massignano section also contains abundant terrestrial chrome-spinel grains, making reconstructions of the micrometeorite flux very difficult. We therefore searched for an alternative section that would be more useful for these types of studies. Here, we report the discovery of such a section, and also the first discovery of the Popigai ejecta in another locality in Italy, the Monte Vaccaro section, 90 km west of Ancona. The Monte Vaccaro section biostratigraphy was established based on calcareous nannoplankton, which allowed the identification of a sequence of distinct bioevents showing a good correlation with the Massignano section. In both the Monte Vaccaro and Massignano sections, the Popigai ejecta layer occurs in calcareous nannofossil zone CNE 19. The ejecta layer in the Monte Vaccaro section contains shocked quartz, abundant pancake spherules, and an iridium anomaly of 700 ppt, which is three times higher than the peak Ir measured in the ejecta layer at Massignano. In a 105-kg-size sample from just above the ejecta layer at Monte Vaccaro, we also found an enrichment of H-chondritic chromite grains. Because of its condensed nature and low content of terrestrial spinel grains, the Monte Vaccaro section holds great potential for reconstructions of the micrometeorite flux to Earth during the late Eocene using spinels.

We present the results of integrated biostratigraphic (planktonic foraminifera, calcareous nannofossils, and dinoflagellates), magnetostratigraphic, and cyclostratigraphic analyses of the lower part of Monte Cagnero section (Umbria-Marche Apennines of Italy), a continuous and complete succession of pelagic limestone and marls that provides the means for an accurate and precise astrochronologic calibration of the Eocene-Oligocene transition. This 38.5-m-thick section overlaps the Oligocene section, which, at meter level 188, contains the Rupelian-Chattian boundary corresponding to the O4-O5 planktonic foraminiferal zonal boundary within the upper half of magnetochron C10n. The Eocene-Oligocene boundary at Monte Cagnero, as defined by the last occurrence of hantkeninid planktonic foraminifers (E14-E15 zonal boundary), is found at meter level 114.1, in the upper part of calcareous nannofossil zone CP16a, and very near the Aal-Gse dinocyst zonal boundary. Paleomagnetic analysis has identified all the magnetic reversals from the lower C13r to the lower C12n, precisely overlapping the base of the Oligocene magnetostratigraphic succession and placing the Eocene-Oligocene boundary in the upper part of C13r, in full agreement with the global stratotype section and point (GSSP) at Massignano. Spectral analysis of calcium carbonate data from bulk samples, collected at 5 cm intervals, indicates that orbital forcing of depositional cycles (i.e., limestone versus marl alternations) is dominant at frequencies corresponding to the theoretical astronomical curves of eccentricity, obliquity, and precessional cycles throughout the studied Eocene-Oligocene transition. Correlation with the astrochronologic time scale allows an age assignment of 33.95 Ma for the Eocene-Oligocene boundary, which is in close agreement with the astrochronologic age for the boundary in the GSSP of Massignano obtained in a similar study by R.E. Brown and colleagues in this volume. Thus, the Monte Cagnero section represents a candidate parastratotype for the Eocene-Oligocene GSSP of Massignano in the eventuality that the oxygen and carbon stable isotope shifts defining the oxygen isotope Oi-1 glaciation will be preferred over the last occurrence of hantkeninids as marker for the boundary, since, at Massignano, the beginning of this isotope shift is barely represented in the uppermost part of the exposed section. The excellent integrated stratigraphic framework of Monte Cagnero presented here will significantly facilitate further high-resolution isotope and paleoecological studies across the time of transition from a hothouse to icehouse Earth.

An organic rich, gleyed soil (Cumulic Haplaquoll) on the edge of a sag pond near San Marino High School illustrates the effects of poor drainage and tectonism on soil formation (pedogenesis) in a semiarid region during the Holocene. Combined with other characteristics of soil development, the organic matter masks the presence of six depositional strata in the granodioritic alluvium in which the soil formed. Two of these units occur only on the south side of a strand of the Raymond fault, three occur only on the north side, and one occurs on both sides. The dominant clay minerals are smectites that have formed from soil solution within the past 10,000 years. Beidellite formed in horizons with exchangeable aluminum (pH less than 6.5), and montmorillonite formed in horizons without exchangeable aluminum (pH greater than 6.5). A soil tongue produced by fault movement was relatively unweathered after it was emplaced by an earthquake that occurred less than 1,400 years ago. Compared to other Holocene soils, pedogenesis at this site is intermediate between the weakly developed Hanford soils formed in granitic alluvium in the San Joaquin Valley and the strongly developed Concepcion soils formed in continental terrace deposits along the coast.